Presently, there are many apparatus which can operate by a power supply from a battery. Among these apparatus, an apparatus capable of being used outdoors has a big issue of a battery life of power supply.
In the following, description will be made by referring to a digital camera as one example of electronic apparatus capable of being used outdoors.
In a generally known digital camera, an object image taken through a photographing is photoelectrically converted by an image pickup device into an image signal, the image signal is A/D converted and recorded on a recording medium, and an image can be displayed on a built-in liquid crystal monitor.
Particularly, a single-lens reflex digital camera capable of exchanging a photographing is required to have a high quality of a photographed image, a wide luminance range of an object capable of being photographed and the like, while a good manipulation performance and a high speed continuous photographing performance are also maintained similar to a silver salt film camera. It is therefore essential that a high sensitivity image pickup device which has a large number of pixels is adopted. In addition, as compared to a silver salt film camera, large scale electronic circuits using a number of electric components are additionally used, including an image pickup circuit, an image processing circuit, an image display circuit and the like. Therefore, a consumption power becomes large and it is required to have a battery capable of supplying a sufficient energy. While compactness and lightness of cameras advance, conventional primary and secondary batteries are becoming difficult to supply cameras with a sufficient drive energy.
In order to solve these problems, a compact fuel cell has been paid attention. In a fuel cell, fuel gas such as hydrogen as a reaction gas is electrochemically reacted with oxidant gas such as oxygen contained in an atmospheric air, to thereby convert chemical energy contained in the fuel, directly into electric energy.
Next, description will be made on a power generation principle of a fuel cell. In a fuel cell, fuel gas containing hydrogen is supplied to a fuel electrode and oxidant gas containing oxygen is supplied to an oxygen electrode to thereby obtain an electromotive force through electrochemical reactions occurring between both the electrodes. Hydrogen supplied to the fuel electrode is separated by catalyst into protons and electrons. The separated electrons move to the oxygen electrode via an outer circuit, whereas the protons move to the oxygen electrode via a solid state polymer film. At the oxygen electrode, protons, electrons and oxygen are coupled to generate water and carbon dioxide. In the following, the electrochemical reactions in the fuel cell are shown. A formula (1) indicates a reaction at the fuel electrode, a formula (2) indicates a reaction at the oxygen electrode, and a formula (3) indicates a reaction in the whole battery.H2→2H++2e−  (1)(½)O2+2H++2e−→H2O  (2)H2+(½)O2→H2O  (3)
Fuel batteries are classified into various types depending upon an electrolyte difference and the like. One know type is a fuel cell which uses a solid state polymer film as electrolyte. A solid state polymer electrolyte type fuel cell can realize low cost, is easy to make compact and light, and has a high output density in view of a battery performance. From these reasons, the fuel cell of this type is desired to be a drive electric power source not only for cameras, but also for portable electronic apparatus such as note type personal computers, mobile phones and PDAs. A stack cell type fuel cell has also been proposed, having a structure that a plurality of power generation cells and separators are alternately laminated.
FIG. 11 is a diagram showing a change in an output voltage of a fuel cell in use. FIG. 12 is a diagram showing a change in an output voltage of a fuel cell in use under purge. As a power is generated by a fuel cell for a long period of time, an output voltage lowers. This is because water generated by a reaction between hydrogen and oxygen diffuses in a reverse direction to the fuel electrode and reduces a power generation area, and in addition, gas unnecessary for power generation remains at the fuel electrode and lowers a hydrogen partial pressure. Since a fuel cell is used as a current supply source of an electronic apparatus, it is not preferable that an output voltage lowers to an allowable voltage range of the electronic apparatus or more.
In order to solve this, generally a flow rate of hydrogen to be supplied to the fuel electrode is increased instantly to discharge moisture and gas which are unnecessary for power generation and remain at the fuel electrode, to an external of stack cells, so that a power generation area is recovered and a hydrogen partial pressure is raised to thereby stabilize an output voltage (a purge method). After purge, an output voltage of the fuel cell rises as shown in FIG. 12.
Several techniques have been disclosed as methods of judging the timing when a fuel cell is purged. According to one method, voltages of all stack cells constituting a fuel cell are detected, and when any one layer of cells presents a predetermined voltage or lower, purge is effected (for example, refer to JP-A-2002-093438). There are a method of forcibly effecting purge each time a predetermined time lapses (for example, refer to JP-A-2000-215905) and a method of performing both periodical purge and hydrogen purge upon voltage measurement of each cell (for example, refer to JP-A-2003-115314). Other methods include a method (for example, refer to JP-A-2002-164065) of measuring voltage and current output from a fuel cell for a predetermined period or by a predetermined number of samples, calculating an internal resistance from the measured value, comparing the internal resistance with a preset standard value, and estimating the state of electrolyte of the fuel cell to effect purge when necessary.
However, although purge is effected to recover the power generation area, raise the hydrogen partial pressure and stabilize the output voltage, an output voltage of the fuel cell lowers momentarily during purge because of air mixture to the fuel electrode by reverse diffusion, drop of hydrogen pressure and temperature in the fuel electrode. A portable electronic apparatus such as a digital camera is required to have a severe precision of management of a power source system. For example, if the timing when the fuel cell is purged is superposed upon the timing when the electronic apparatus requires a relatively large power, the power necessary for the electronic apparatus cannot be drawn from the fuel cell, and there arises a problem of an insufficient power of the electronic apparatus.
If an electronic apparatus is remained unused for a long term, fuel gas leaks minutely from the fuel electrode of a fuel cell, so that a hydrogen concentration on the fuel electrode side falls. When the fuel cell is thereafter activated, there arises a problem that it takes a long time to raise the output voltage to a necessary voltage.